There's another layer of complexity here that you're glossing over, I think, and that's class dynamics within the Maori population.

It could both be true that traditional Maori lifeways were more sustainable, and that modern, Maori owned fishing companies are over fishing. Let's assume all of that is correct for a minute

The coming of the white man didn't ruin the sustainability of fishing, because of something ontologically bad with white people, but because they enforced an extractive, capitalist, economic system onto the region.

Colonialism pulled the Maori into a broader world system which generated a group of Maori with enough capital to, say, found fishing companies, and a wide swathe of Maori who can't.

And paradoxically, that capital generation from unsustainable, capitalist, fishing practices is probably one of the things that allows Maori communities to have a degree of sovereignty, all the while said fishing practices are undermining their ability to continue to sustain themselves.